Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some non-volatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen species. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline-blending stocks diminishes.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air in a regenerator separate from the fluidized reactor used in catalytic cracking. In the catalyst regenerator, the coke burns off, restoring catalyst activity and heating the catalyst to, e.g., 500-900° C., usually 600-750° C. Flue gas formed by burning coke in the regenerator may be treated to remove particulates and convert carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
The removal of carbon monoxide from the waste gas produced during the regeneration of deactivated cracking catalyst can be accomplished by conversion of the carbon monoxide to carbon dioxide in the regenerator or carbon monoxide boiler after separation of the regeneration zone effluent gas from the catalyst.
Initially, there was little incentive to attempt to remove substantially all coke carbon from the catalyst, since even a fairly high carbon content had little adverse effect on the activity and selectivity of amorphous silica-alumina catalysts. Most of the FCC cracking catalysts now used, however, contain zeolites, or molecular sieves. Zeolite-containing catalysts have usually been found to have relatively higher activity and selectivity when their coke carbon content after regeneration is relatively low. An incentive arose for attempting to reduce the coke content of regenerated FCC catalyst to a very low level.
When the regenerators operate in a complete CO combustion mode, the mole ratio of CO2/CO is at least 10 in the regenerator flue gas. During regeneration operated at complete combustion mode, several methods have been suggested for burning substantially all carbon monoxide to carbon dioxide to avoid air pollution, recover heat, and prevent afterburning. Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regeneration have been: (1) increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either (2) increasing the average operating temperature in the regenerator or (3) including various carbon monoxide oxidation promoters in the cracking catalyst to promote carbon monoxide burning. Various solutions have also been suggested for the problem of afterburning of carbon monoxide, such as addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat of combustion of carbon monoxide.
Specific examples of treatments applied to regeneration operated in the complete combustion mode include the addition of a CO combustion promoter metal to the catalyst or to the regenerator. For example, U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121 taught using relatively large-sized particles containing CO combustion-promoting metal into a regenerator. The small-sized catalyst is cycled between the cracking reactor and the catalyst regenerator while the combustion-promoting particles remain in the regenerator. Also, U.S. Pat. Nos. 4,072,600 and 4,093,535 teach the use of Pt, Pd, Ir, Rh, Os, Ru, and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory to promote CO combustion in a complete burn unit. Most FCC units now use a Pt CO combustion promoter. While the use of combustion promoters such as platinum reduce CO emissions, such reduction in CO emissions is usually accompanied by an increase in nitrogen oxides (NOx) in the regenerator flue gas.
It is difficult in a catalyst regenerator to completely burn coke and CO without increasing the NOx content of the regenerator flue gas. Many jurisdictions restrict the amount of NOx that can be in a flue gas stream discharged to the atmosphere. In response to environmental concerns, much effort has been spent on finding ways to reduce NOx emissions.
For example, NOx is controlled in the presence of a platinum-promoted complete combustion regenerator in U.S. Pat. No. 4,290,878, issued to Blanton. Recognition is made of the fact that the CO promoters result in a flue gas having an increased content of nitrogen oxides. These nitrogen oxides are reduced or suppressed by using, in addition to the CO promoter, a small amount of an iridium or rhodium compound sufficient to convert NOx to nitrogen and water.
U.S. Pat. No. 4,300,997 to Meguerian et. al discloses the use of a promoter comprising palladium and ruthenium to promote the combustion of CO in a complete CO combustion regenerator without simultaneously causing the formation of excess amounts of NOx. The ratio of palladium to ruthenium is from 0.1 to about 10.
As opposed to complete CO combustion, older FCC catalyst regeneration techniques are operated in an incomplete mode of combustion or in “partial burn” units. This invention is concerned with such modes of operation rather than the complete CO combustion mode described immediately above. Incomplete CO combustion modes of operation are usually referred to as “standard regeneration” wherein a relatively large amount of coke is left on the regenerated catalyst which is passed from an FCC regeneration zone to an FCC reaction zone. The relative content of CO in the regenerator flue gas is relatively high, i.e., about 1 to 10 volume percent. The concentration of carbon is approximately 0.25 to 0.45 weight percent relative to the regenerated catalyst. Under incomplete combustion operation NOx is not observed in the regenerator flue gas, but sizable amounts of ammonia and HCN are present in the flue gas. According to U.S. Pat. No. 4,744,962, the regenerator flue gas formed under incomplete combustion typically comprises about 0.1-0.4% O2, 15% CO2, 4% CO, 12% H2O, 200 ppm SO2, 500 ppm NH3, and 100 ppm HCN. If the ammonia and HCN are allowed to enter a CO boiler, much of the ammonia and HCN will be converted to NOx.